Some ECUs are cross flashable - meaning you can use the flash from another ECU on them, e.g. people use the 551M flash on most other 2.7T ecus.

There is not an easy way to figure out what you can and cant cross flash, other than a lot of reading on here. For starters, 512kb and 1024kb ECUs are definitely not cross flashable, the different versions of motronic (7.1, 7.1.1, 7.5) are not cross flashable.

You probably want to look at other AUQ ecus, such as an 032HN, if you cannot find one then look at other 1.8t ECUs - there are a lot of threads about compatibility where people have cross flashed stuff.

See here:
http://nefariousmotorsports.com/forum/index.php?action=printpage;topic=2031.0
etc

My AUQ is using an ECU from a 2001 A6 1.8T (4B0906018DC) with no issues.

MJ and HN can be crossflashed

I believe what the man is saying is they are compatible from a hardware standpoint. Most likely VAG100. Here is a list of those ECUs from an old post on this site:

VAG100
VW Golf,Jetta NA MY2002 1.8T 06A906032HS S0006
VW Golf,Jetta NA MY2002 6-spd 1.8T 06A906032ML S0001
VW Golf,Bora EURO MY2002 1.8T 06A906032HJ S0002
VW Golf,Bora EURO MY2003 1.8T 06A906032KP S0203
VW Golf,Bora EURO MY2001 1.8T 06A906032EN S0002
VW Golf,Jetta NA MY2002 Auto 1.8T 180hp 06A906032HF S0005
Audi A3, Seat Leon Cupra EURO MY2002 1.8T 180hp 06A906032HN S0001
Audi A3, Seat Leon Cupra EURO MY2002 1.8T 06A906032HK S0002
Audi A3, Seat Leon Cupra EURO MY2003 1.8T 06A906032HR S0001
Audi A3, Seat Leon Cupra EURO MY2002 1.8T 06A906032HP S0001
Audi TT NA MY2002 1.8T 180HP 8N0906018AL S0004
Audi TT EURO MY2001 1.8T 180hp 8N0906018AQ S0001
Audi TT EURO MY2002 1.8T 180HP 8N0906018AM S0001
Audi TT EURO MY2003 1.8T 180HP 8N0906018AP S0001

If the Big D says that your MJ should work with the HN, then these should too. I have no direct experience other than by crossflashing HS and HN (edit: And HJ, forgot that). No issues there.

And no it is not plug and play. You will have to flash your .bin and eeprom to the new box. The eeprom is what contains your immo data.

I did what you are proposing. I found an Ebay ECU and then flashed a copy of my ori bin and eeprom to it. Put it in the car and no problems, immo or otherwise. I've been tuning on it exclusively. My original is sitting safe on my bookshelf... For the day I really screw up this one.

Thanks a lot both of you!!!